How to fix burned traces?

[gnubag]

I just recieved a KBT ONi where one of the traces of the right alt key are burned from a friend. With that key all modifiers on the left side and the spacebar don't work anymore.

How can I find out where the trace goes to fix those keys since i can't figure out where the traces go? (I assume testing with a muiltimeter)

it works now for some reason.... i dont know why...

[mkawa]

one needs pictures to solve this conundrum. eg, can you see the broken trace? if so, it should be pretty easy to fix. if not, then yep, you'll need a continuity tester.

[gnubag]

here are some pictures: http://imgur.com/a/M7vMK

i will take some better ones tomorrow with more/better light if needed.
hmmm.. they got downscaled a bit now that i checked. I will upload higher res pictures later.

[gnubag]

hm... i can't upload higher res pictures, but i am getting a multimeter lend tomorrow.

what am i looking for?
what do I have to test for?

[mkawa]

very difficult to see from these pictures (out of focus!)

what it looks like is that someone soldered a lot of cold joints all over this board. there's flux everywhere and a lot of really dark joints. a good joint is shiny and the solder is evenly distributed across the joint (because the forces on the molten metal are isometric as it cools back into a solid). what i would do is start by just reflowing all the solder joints. if there's too much solder on the joints (some of those look like they had have a spool melted onto the joint..), then suck off as much solder as you can first, then resolder the board. once you have lots of nice shiny joints, start looking for physical damage to the board. if the pcb was burned it will be pretty obvious. i don't see any obvious damage to the pcb though. all i see are lots of joints that probably don't conduct much if any electricity.

[gnubag]

very difficult to see from these pictures (out of focus!)

what it looks like is that someone soldered a lot of cold joints all over this board. there's flux everywhere and a lot of really dark joints. a good joint is shiny and the solder is evenly distributed across the joint (because the forces on the molten metal are isometric as it cools back into a solid). what i would do is start by just reflowing all the solder joints. if there's too much solder on the joints (some of those look like they had have a spool melted onto the joint..), then suck off as much solder as you can first, then resolder the board. once you have lots of nice shiny joints, start looking for physical damage to the board. if the pcb was burned it will be pretty obvious. i don't see any obvious damage to the pcb though. all i see are lots of joints that probably don't conduct much if any electricity.

so here are new pictures (focuesd this time)

the solder joints are not the problem (they are pretty shiny it's just the light)
the pcb worked without a problem until the soldering at the right alt key where the traces got burned. all keys work except for the left modifiers, spacebar and right alt key.

[mkawa]

i still can't for the life of my tell what traces were damaged from those pictures. can you extract all the solder, scrub the area down and take a picture of a normal joint in that position vs the damaged area? basic idea is: if there is damage to a surface trace, use a conductive silver ink pen (available from all the major chem vendors). if there is damage to a solder pad, affix a new pad with a permanent adhesive and then use a jumper wire to electrically join the new pad to something it should be connected to electrically. if it's a multi-layer pcb and your bud managed to burn out a trace on a lower layer, figure out what the circuit is supposed to look like electrically and again, use a jumper wire to join two disconnected joints that should be connected.

when doing this kind of work, you need to have a couple basic tools: solder sucker and braid (soldapult and mg chem rosin braid are my faves), iron (hakko 888 is my fave, but temp control isn't absolutely necessary), and a basic multimeter. a surplus no-name is fine. fancier multimeters are more precise/calibratable, and hold that precision despite being beat upside the head daily.